This invention relates to the design of a bow and hull for an icebreaker designed to move through an ice field typically located in a polar region.
Ships designed for breaking channels in ice-covered waters have changed and improved over the decades but have always retained common characteristics and structural details to which are added the new and more effective hull forms and designs. In general, all icebreakers incorporate a section at the bow that differs from the typical deep V-shaped or U-shaped sections for non-icebreaking ships by reason of the bow being flatter in areas that are designed to contact the ice.
In one form of icebreaker, the bow is spoon-shaped, as shown in U.S. Pat. No. 4,702,187, for example. Relatively low resistance to breaking level ice is achieved with such a spoon-shaped bow but in the earlier days of use, there was insufficient power capability in the vessels to achieve the maximum benefits and efficiency from such a bow particularly in ice-clogged channels. Thus, the icebreaker bows were designed with a sharper angle that would move more easily in ice-clogged channels. As the ships were fitted with more powerful engines, the icebreakers were widened as well and, using relatively shallow drafts, the bow sections were flattened resulting in improved icebreaking capability and good advance in ice-clogged channels.
These previous designs had some success but a successful and efficient design for any icebreaker necessarily must consider the removal of the broken ice in the path of the ship. Ice may be broken by either bending, shearing or crushing. Bending is found to be the most common way of icebreaking using the downward force resulting from the weight of the ship. Crushing is not particularly efficient because the strength of the ice against crushing is considerably greater than against bending. Shearing is the most energy efficient wa of breaking ice but requires special hull forms and bow shapes.
The stem angle of a conventional icebreaker is usually the initial factor to consider because the stem angle determines the vertical force for bending the ice. A small stem angle maximizes the vertical force bending the ice. The average stem angle, that is the angle of the straight stem line with respect to the waterline, of conventional icebreakers has typically been in the range of 20.degree.-30.degree. with initial entrance angles at the design waterline as low as 15.degree.. A step beyond this is a bow with an S-shaped stem line. This provides a low angle near the design waterline facilitating the breaking of level ice and an increased angle near the forefoot allowing the ship to ride up quicker onto the ice and slide off the ice more easily while ramming in heavy ice conditions. Such bow design, known as the North American White bow, is in use today on many icebreakers.
Most bows have been designed using the weight of the bow to bend the ice into breaking. This bending failure of the ice is found to generate cusps of ice that are rotated by the hull and pushed out of the path of the ship. The cusps are larger in thicker ice and are generated not only at the bow but all along the waterline to the point of maximum beam in level ice. Low icebreaking resistance hull forms often force the broken pieces downwardly. These pieces adhere to the hull by suction and tend to move slowly toward and through the ship's propellers and then into the broken channel behind the ship. The milling of these ice cusps by the propellers seriously reduces the performance of the propellers and the vessel requires additional power for the icebreaker to continue to meet the design capabilities of the ship.
Numerous methods have been tried and tested to achieve more effective control of the ice cusps after they have been broken and forced to the side of the ship. Ideally, such cusps should be forced to the side under the unbroken ice and not passed downward toward the stern of the ship and into the flow of water into the propellers.
As noted, spoon bow forms have been shown to have lower resistance to icebreaking than the White bow. Most spoon bow forms incorporate a straight stem at a low angle with straight parallel buttocks, a convex waterline and a large lateral radius to the stem. Compared to more wedge-shaped stems, however, spoon bows are at a disadvantage. A sharper stem is better in ridge ramming and provides directional stability during ramming and breaking out of an existing channel.
A number of specific bow shapes are well known to the art that are capable of maximizing certain characteristics for effective icebreaking but do not meet all of the requirements for efficient and effective icebreaking in polar regions. Of particular importance to this challenge are the following:
1. minimize the resistance to icebreaking in level ice of significant thickness;
2. provide good ramming capability and directional stability for breaking out of channels;
3. produce a clear channel behind the icebreaker;
4. minimize the amount of broken ice that reaches and contacts the propellers of the icebreaker.
To date, the prior art shows no effective hull and bow form for a polar icebreaker that both provides an effective icebreaking capability and controls the flow of ice away from the propellers so as to provide maximum icebreaker thrust.